This section provides background information related to the present disclosure which is not necessarily prior art.
With the popularity of smart phones, increasingly enlarged display panels are presently applied in the smart phones, and the smart phones have become increasingly thinner. Due to its complicated structure, the traditional moving coil receiver (i.e. telephone receiver) is hard to reduce in size and thickness. Furthermore, the traditional moving coil receiver includes a vibrating membrane which can cause the air to vibrate to generate sound waves, and the traditional moving coil receiver is required to be provided with an opening for air vibration conduction, thereby causing an inconsistent appearance of the moving coil receiver and raising the risk of dust or moisture permeation.
The local sound pressure level in an air-conduction receiver is generally ranged from 90 dB to 100 dB in operation, and the sound pressure level is lowered to a range of 50 dB to 60 dB at a distance of one meter away from the air-conduction receiver in a quiet environment (such as in an office space with a noise level of 50 dB), whereby telephone conversations can be easily perceived by the people around to cause privacy leakage.
Piezoelectric materials are special substances with a unique property. When piezoelectric crystals or piezoelectric materials are deformed by an applied external force, electrical charges which are variable depending upon the state of the applied external force are accumulated on outer surfaces of the piezoelectric crystals or the piezoelectric materials, thereby causing the piezoelectric crystals or the piezoelectric materials to exhibit the direct piezoelectric effect. When an external electric field is applied to piezoelectric crystals or piezoelectric materials, the piezoelectric crystals or the piezoelectric materials are mechanically deformed, and an amount of the deformation is variable depending upon the state of an intensity of the applied electric field, thereby causing the piezoelectric crystals or the piezoelectric materials to exhibit the reverse piezoelectric effect. With the above-mentioned unique property, the piezoelectric materials have been applied in the speaker and the touch panel.
Acoustic vibration may be transferred to the auditory nerve in two manners, i.e. an air-conduction manner which is almost dominant in acoustic vibration propagation and a bone conduction manner which is generally negligible for forming a hearing. The bone conduction manner, in which the sound waves are transferred into the inner ear via skull rather than via the external auditory canal and the eardrum, includes a translatory mode of bone conduction and a compressional mode of bone conduction, and a synergistic action between the translatory mode of bone conduction and the compressional mode of bone conduction can stimulate the Corti's organ to form a hearing.
A conduction path of the bone conduction manner is described as follows:
The sound waves-skull-bony labyrinths-inner ear lymphs-Corti's organ-the auditory nerve-the brain auditory Hub.
The currently available bone conduction device usually contacts with at least one of various body parts such as the teeth, the shoulder, the otica, the afterbrain and the mastoid process in use.
A piezoelectric transducer includes, as an acoustic vibration generating element, a piezoelectric bimorph or unimorph element. The piezoelectric bimorph or unimorph element includes a metal plate and a piezoelectric plate adhered to the metal plate. As mechanical vibration is variable depending upon the state of a piezoelectric deformation caused by voltage rather than current, the piezoelectric transducer is not accompanied with dissipation of Joule heat by the coil in the electromechanical transducer of the electromagnetic type. In addition, due to the absence of the magnet and the magnetic conductive bowl, the piezoelectric transducer becomes small in size and light in weight.
In Chinese Patent Application Publication No. CN1627864A (which was filed on Dec. 1, 2004, in the name of NEC TOKIN CORPORATION (JP), and relates to a piezoelectric device), a piezoelectric bone conduction speaker is described, wherein a piezoelectric unimorph element functioning as an acoustic vibration element includes a metal plate and a piezoelectric ceramic plate adhered to the metal plate, and a flexible silicone material is coated on an outer surface of the piezoelectric unimorph element to improve the frequency response performance of the piezoelectric unimorph element.
The piezoelectric bone conduction device as described In Chinese Patent Application Publication No. CN1627864A has a relatively high impedance characteristic and a relatively high resonance frequency characteristic, and requires a relatively high input driving voltage, so that sound reproduction in the low-frequency region lower than the resonance frequency is insufficient. Moreover, generation of vibration at a specific frequency is likely to be enhanced or weakened due to a high mechanical quality factor of the vibration system, thus sound reproduction cannot naturally and normally be carried out.